This invention relates to air depolarized alkaline electrochemical cells. Typically, such cells have metal-containing anode materials, and air cathodes, and are commonly known as metal-air cells. More particularly, this invention relates to the composition and structure of cathode cans utilized in such cells, and in general to the cells, themselves. The invention addresses the efficiency of use of the three-dimensional volume available, in electrical appliances, for use by such cells. The invention particularly addresses efficient use of non-reactive e.g. structural material in preserving as much space as possible for occupation by the electrochemically reactive anode material used by the cell for generating electrical energy. Increased efficiency of use of non-reactive material provides an increase in the fraction of the overall volume of the cell which can be allocated to, or occupied by, the electrochemically reactive anode material.
The growth in use of small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Metal-air cells have gained significant popularity because only the anode reaction material need be packaged in the cell. In combination, the cathode reaction material is oxygen, which is drawn from the surrounding ambient environment.
Such small cells are usually disc-like or pellet-like in appearance, and are about the size of garment buttons. These cells generally have diameters ranging from less than 5.8 millimeters to about 25 millimeters, and heights ranging from less than 2.0 millimeters up to about 15 millimeters. The small size of such cells, and the limited amount of electrochemically reactive material which can be contained in such small metal-air cells, result in a need for improving the efficiency and completeness of the electrochemical reactions, which are used in such cells for generating electrical energy, and for improving the fraction of the overall volume of such cell which can be occupied by the electroactive anode material.
Such metal-air cells take in atmospheric oxygen, and convert the oxygen to hydroxyl ions in the air cathode by interaction with aqueous alkaline electrolyte. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc, although a variety of other operable anode materials are well known to those skilled in the art.
More particularly, the desired reaction in the air cathode of a metal-air cell involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions. The hydroxyl ions migrate through the aqueous alkaline electrolyte toward the anode, where oxidation occurs, forming zinc oxide.
In typical metal-air cells, air enters the cell through one or more air ports in the bottom the cathode can. The port or ports extend through the bottom wall of the cathode can, and may be immediately adjacent the cathode assembly, or may preferably be separated from the cathode assembly by an air reservoir, which is typically occupied by an air diffusion member.
In such arrangements, the port facilitates movement of air through the bottom of the cathode can and to the cathode assembly. At the cathode assembly, the oxygen in the air reacts with water as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms the hydroxyl ions.
Since the overall electrochemical capacity of any electrochemical cell is to some extent determined by the quantity of electrochemically reactive materials which can be loaded into the cell, it is important to maximize, in the cell, the size of the cavity which is devoted to containing the electrochemically reactive materials. In the case of a metal-air cell, contained reactive material is limited to the anode material.
In general, the size of any given cell is limited by the inside dimensions of the space provided in the article, namely the appliance, in which the cell will operate. For example, the size of a hearing aid cell is limited to the internal dimensions of the space, provided for the cell, in the hearing aid appliance. The internal dimensions of the space are determined by the hearing aid manufacturer, not the power cell manufacturer.
Thus, any given appliance includes a limited amount of gross space or volume allotted to occupancy by the electrochemical cell which powers the appliance. That gross space may ultimately be divided according to four functions, all competing for portions of the gross space. A first portion of the space is used to provide clearance between the interior elements of the space and the exterior elements of the electrochemical cell.
A second portion of the space is occupied by the structural and otherwise non-reactive elements of the electrochemical cell.
A third portion of the space is allocated for occupation by the electrochemically reactive material in the electrochemical cell, and, in a metal-air cell, especially the anode material.
Finally, a fourth portion of the space, as appropriate, can sometimes be described as xe2x80x9cwastedxe2x80x9d space, because it serves none of the above first through third functions. Such wasted space is typically found outside the cell, e.g. at corner locations, where the corner of the cell is less square than is structurally feasible, thereby wasting volume that potentially might be occupied, either directly or indirectly by electrochemically reactive material. Such wasted space might also be considered to be included in the space allocated to clearance because such space is typically located outside the cell.
Any increase in the third portion of the space, namely the cavity in the anode can which cavity is allocated to the anode material, is necessarily gained at the expense of one or more of the other three portions of the fixed volume allocated for occupation by the cell, namely the first clearance portion, the second portion devoted to the non-reactive elements of the cell, or any fourth waste portion. Thus, it is important to identify the first, second, and fourth portions of the overall space, and, where possible, to reduce the amount of space devoted to such uses. To the extent such uses can be reduced, the space so recovered can, in general, be allocated for use to hold additional amounts of electrochemically reactive anode material, thereby increasing the potential overall capacity of the cell to generate electrical energy within the limited amount of gross space or volume provided, in the appliance, for occupation by the cell.
Overall cell height and width dimensions are specified by the International Electrotechnical Commission (IEC).
Of the first, second, and fourth portions of the cell, the opportunity for capturing space from the first portion, devoted to clearance, relates in part to the ability of the manufacturer to control the range of outer diameters of the cathode cans from which the cells are made. To the extent the range of diameters can be reduced, nominal clearance may be reduced accordingly.
It is known that traditional methods of forming cathode cans for use in hearing aid cells have a tendency to form an outward bulge in the diameter of the cathode can at the intersection of the side wall with the bottom wall, whereby allowance must be made in the can specification for occurrence of such bulge.
In addition, applicants have concluded that further potential for recovering space for use in holding anode material, and thus to increase volume efficiency of the cell, lies primarily in the second portion of the cell, namely the structural and otherwise non-reactive elements of the cell. These elements generally comprise the cathode can, the anode can, the seal, and the cathode assembly, these typically representing all of the major structural elements of the cell. Thus, to get more space from the second portion of the cell, that space must be taken from the anode can, the cathode can, the cathode assembly, or the seal, or some combination of these.
This invention focuses on apparatus, methods, and materials for providing improved cathode cans, and wherein the cathode cans have reduced cross-section thicknesses, and reduced range of thicknesses from can to can, while maintaining suitable strength parameters to properly support the manufacture and use of such cans, and cells made therewith. Such cans typically have a pair of nickel layers, and a steel layer between the nickel layers.
It is known to desire to reduce the thickness of the non-reactive structural materials of the cell. However, the desire to reduce the thickness of such non-reactive elements operates in tension against the requirement that such structural elements have suitable fabrication properties, and suitable strength to support the fabrication and use of the cell. Accordingly, an element cannot simply be made thinner without considering the effect such thinning will have on the ability to fabricate the element, or to fabricate and use a cell made therewith.
Similarly, it is known to select different materials from which to fabricate the respective non-reactive elements. Such different materials may have different chemical composition, or different chemical or physical properties. However, changing material selection also affects the ability to fabricate the element, and the ability of the element to support fabrication and use of the cell.
Thus, where thinner, or harder, metal strip is contemplated for use to form cathode cans, there is the prospect of developing cracks in one or more of the layers of the metal strip during conventional fabrication of the can.
Accordingly, any change in selection of material from which the cans are to be made, or physical dimensions of such material, must be carefully balanced against the fabrication requirements associated with such material as the material is used to fabricate the respective elements; as well as the requirements associated with fabrication and use of a cell utilizing such elements. Any change of material must, of course, be compatible with the chemical operating environment within which the cell operates. Typically, air depolarized cells operate in an alkaline environment, and so any material used therein must be compatible with such environment.
It is an overall object of the invention to provide improved air depolarized electrochemical button cells.
It is a more specific object of the invention to provide cathode cans wherein the side walls are harder than the bottom walls.
It is yet another object to provide cathode cans wherein outwardly-disposed side walls have smoother finishes than corresponding surfaces of the respective bottom walls.
It is still another object to provide can forming systems including a punch and a die wherein the clearance between the punch and die is less than the thickness of a metal strip to be formed therebetween.
Yet another object is to provide methods of making cathode cans in a metal strip, wherein the clearance between the punch and the die is less than the thickness of the metal strip.
It is a further object to provide cathode cans wherein the side wall is harder than, and thinner than, the bottom wall, and wherein an outwardly-disposed surface of the side wall has a smoother finish than a corresponding outwardly-disposed surface of the bottom wall.
The invention, in general, comprehends a cathode can, for use in an air depolarized electrochemical cell, the cathode can comprises a bottom wall, and a circumferential side wall, extending upwardly from a lower edge of the side wall adjacent the bottom wall, and terminating at an upper, distal edge. The side wall has a height generally corresponding to an overall height of the cathode can of no more than about 15 mm, preferably no more than about 8 mm, and a circumference defining an overall diameter of the cathode can of no more than about 25 mm, preferably no more than about 13 mm. The ratio of the overall height to the overall diameter of the cathode can is about 0.1/1 to about 1/1. The side wall has a first strength, as measured by hardness, greater than a second strength, as measured by hardness, of the bottom wall. The strength of the side wall is related to the strength of the side wall as a side wall hardness of about 130 to about 185 Vickers (84-90 on the Rockwell Hardness 15T scale) is related to a bottom wall hardness of about 93-117 Vickers (77-82 on the Rockwell 15T scale).
The ratio of the hardness of the side wall to the hardness of the bottom wall, on the Vickers scale, is preferably between about 0.60/1 and about 0.85/1.
Preferred absolute hardness of the side wall is about 130 to about 185 on the Vickers scale and about 84 to about 90 on the Rockwell Hardness 15T scale; and preferred hardness of the bottom wall is about 93 to about 117 on the Vickers scale and about 77 to about 82 on the Rockwell 15T scale.
The cathode can side wall preferably has an outwardly-disposed ironed surface. The outwardly-disposed side wall surface, as ironed, comprises a first surface finish. The bottom wall has a second surface finish. The surface finish of the side wall is preferably related to the surface finish of the bottom wall as a surface finish RA of less than 2 microinches is related to a surface finish RA of about 2 microinches to about 5 microinches.
In preferred embodiments, the bottom wall has a first thickness, the side wall having a second thickness no more than about 85 percent as great as the thickness of the bottom wall.
Preferred embodiments of the can comprise first and second layers comprising nickel, and a layer of steel between the nickel layers, and can include a metal plating layer on at least one of the nickel layers such that the respective nickel layer is between the plating layer and the steel layer.
For use in air depolarized electrochemical cells, a respective cathode can includes at least one air port in the bottom wall.
The invention further comprehends an air depolarized electrochemical button cell having an overall height of no more than about 15 mm, preferably no more than about 8 mm, and a circumference defining an overall diameter of the cathode can of no more than about 25 mm, preferably no more than about 13 mm, the cell comprising an anode assembly, a cathode including a cathode can having a hardened side wall as described above, a separator, and an electrolyte.
In a second family of embodiments, the invention comprehends a cathode can having a bottom wall, and a circumferential side wall, extending upwardly from the bottom wall. The side wall has a height generally corresponding to an overall height of the cathode can of no more than about 15 mm, preferably no more than about 8 mm, and a circumference defining an overall diameter of the cathode can of no more than about 25 mm, preferably no more than about 13 mm. The ratio of the overall height to the overall diameter of the cathode can is about 0.1/1 to about 1/1. The side wall has an outwardly-disposed ironed surface. The side wall surface, as ironed, comprises a first surface finish. The corresponding outer surface of the bottom wall has a second surface finish. The surface finish of the side wall is related to the surface finish of the corresponding outwardly-disposed surface of the bottom wall as a surface finish RA of less than 2 microinches is related to a surface finish RA ranging from about 2 microinches to about 5 microinches.
In some embodiments, the surface finish of the side wall ranges from about RA 0.5 microinch to about RA 1.5 microinches and the surface finish of the bottom wall ranges from about RA 2.5 microinches to about 4.5 microinches.
In preferred embodiments, the cathode can comprises first and second layers comprising nickel, and a layer of steel between the nickel layers, and may optionally include a metal plating layer on at least one of the nickel layers such that the respective nickel layer is between the plating layer and the steel layer; and typically includes at least one air port in the bottom wall.
This second family of embodiments further comprehends an air depolarized electrochemical button cell having an overall height of no more than about 15 mm, preferably no more than about 8 mm, and a circumference defining an overall diameter of the cathode can of no more than about 25 mm, preferably no more than about 13 mm, the cell comprising an anode, a cathode including a cathode can having a surface finish as described above, a separator, and an electrolyte.
A third family of embodiments of the invention comprehends a method of forming a cathode can from a metal strip having a first thickness between opposing surfaces thereof, using a punch in combination with a female die. The female die comprises an initializing land, an upstanding inner side wall, a cavity defined inwardly of the inner side wall, and a lip between the initializing land and the inner side wall. The method comprises urging the punch against an element of the metal strip and thus urging both the punch and the metal strip into the cavity in the female die such that the metal strip is disposed between a first outer surface of a side wall of the punch, and a second inner surface of the side wall of the female die. The metal is thus drawn about the lip of the female die. The lip of the female die comprises a first outer cross-sectional radius disposed toward the initializing land, and a second inner cross-sectional radius disposed toward the inner side wall. The first radius is disposed between the second radius and the initializing land. The second radius is smaller than the first radius and is disposed between the first radius and the inner side wall.
This embodiment further comprehends moving the punch, and the corresponding element of the metal strip, into the cavity such that the outer side wall of the punch comes into facing, and thus working, relationship with the inner side wall of the female die. The clearance between the respective inner and outer side walls is less than the thickness of the metal strip being drawn therebetween, whereby movement of the punch into the cavity and corresponding drawing of the metal strip, along with the punch, and into sliding engagement against the inner surface of the side wall, results in rubbing, surface-to-surface engagement of an outwardly-disposed surface of the metal strip against corresponding portions of the inner surface of the female die, thus drawing the metal strip, and working the surface of the metal strip, thereby making a cathode can precursor as an integral part of the metal strip, the cathode can precursor having a bottom wall, and a side wall extending upwardly from the bottom wall.
This embodiment further comprehends, subsequent to the moving of the punch into the cavity, severing the cathode can precursor from the metal strip, thereby to form the cathode can.
Leading and trailing edges of the element, or workpiece, being worked are cut transversely across the metal strip before the metal strip is urged into the die cavity, while retaining attachment of the element to the metal strip at opposing sides of the strip.
In general, the moving of the punch into the cavity works the metal strip by both thinning the metal and bending the metal. Such preferably cold working of the metal strip at the outwardly-disposed surface increases the smoothness of the outwardly-disposed surface of the metal strip.
In preferred embodiments, the bottom wall of the cathode can precursor has a second thickness, and the side wall of the cathode can precursor has a third thickness, of about 60 percent to no more than about 85 percent, preferably about 60 percent to about 80 percent, as great as the second thickness.
In some embodiments, the metal strip comprises first and second layers comprising nickel, and a layer of steel between the nickel layers, and can further include the step of post-plating the cathode can with a plating material, for example and without limitation, nickel, gold, or silver, after the severing of the cathode can precursor from the metal strip, whereby the worked, outwardly-disposed surface is plated with the plating material.
Preferably, the ratio of the second radius to the first radius is about 2/1 to about 8/1, more preferably about 3/1 to about 6/1, and most preferably, about 4/1.
The ratio of the clearance between the punch and the female die to the thickness of the metal strip, before any working of the metal strip in the invention is about 0.5/1 to about 0.85/1.
The metal strip has a preferred hardness of about 93 to about 117 on the Vickers scale prior to being worked in said cavity.
Preferably, that portion of the metal strip which is worked in the cavity has a worked hardness of about 130 to about 185 on the Vickers scale.
The invention further comprehends an air depolarized electrochemical button cell comprising an anode assembly, a cathode including a cathode can fabricated according to an above-recited method, a separator, and an electrolyte.
In a fourth set of embodiments, the invention comprehends a can forming system for forming a cathode can having at least one air port in a bottom wall thereof, from a metal strip. The can forming system comprises a punch in combination with a female die, and a severing device. The female die comprises an initializing land, an upstanding inner side wall extending about a cavity, and a lip between the initializing land and the inner side wall. The lip of the female die comprises a first outer cross-sectional radius disposed toward the initializing land, and a second inner cross-sectional radius disposed toward the inner side wall. The first radius is between the second radius and the initializing land. The second radius is smaller than the first radius, and is disposed between the first radius and the inner side wall. The severing device severs the cathode can precursor from the metal strip, thereby to form the cathode can.
The can forming system can further include the metal strip, having a thickness, and being disposed between the punch and the die as the punch, and the corresponding element of the metal strip, moves into the cavity such that the outer side wall of the punch comes into facing, and thus working, relationship with the inner side wall of the female die. The clearance between respective inner and outer side walls is less than the thickness of the metal strip being drawn therebetween. Thus, movement of the punch into the cavity and corresponding drawing of the metal strip, along with the punch, and into sliding engagement against the inner surface of the side wall results in rubbing, surface-to-surface engagement of an outwardly-disposed surface of the metal strip against corresponding portions of the inner surface of the female die, thus drawing the metal strip and thereby substantially thinning the metal strip, working the metal strip by both substantial thinning of the metal and bending of the metal. The drawing, working, and corresponding thinning of the metal strip gives the outwardly-disposed surface of the metal strip a finer surface finish, and makes a cathode can precursor as an integral part of the metal strip.
The can forming system commonly includes the metal strip having first and second layers comprising nickel, and a layer of steel between the nickel layers.
The metal strip preferably has a hardness of about 93 to about 117 on the Vickers scale prior to being worked by the can forming system.
In preferred embodiments, that portion of the metal strip which is worked in the can forming system has a worked hardness of about 130 to about 185 on the Vickers scale.
Typical cathode can made with the above can forming system comprises a bottom wall, having at least one air port therein, and a circumferential side wall extending upwardly from the bottom wall, the side wall having a height generally corresponding to an overall height of the cathode can of no more than about 15 mm, preferably no more than about 8 mm, and a circumference defining an overall diameter of the cathode can of no more than about 25 mm, preferably no more than about 13 mm.